According to Faraday's law of induction, when an alternating current (AC) flows through a conductor, that alternating current creates a magnetic field around the conductor due its periodically changing direction. If another conductor, which can be referred to as a secondary conductor, is placed in the vicinity of this magnetic field, current is induced in the secondary conductor. This current is also alternating in nature. Due to the electrical resistance of the secondary conductor to the flow of current, some amount of energy is dissipated as heat. This heat can be harvested for a variety of uses, such as in the well-known induction stove top.
In an induction stove top, the induction stove top itself contains a resonant coil through which alternating current flows when the induction stove top is activated. Cookware made from a ferromagnetic material, such as stainless steel or iron, is placed on the induction stove top.
This system of the resonant coil and cookware can be considered as a transformer in which the cookware acts as a shorted secondary (load). As stated, alternating current flows through the resonant coil when the induction stove top is activated, which results in the generation of an oscillating magnetic field. This oscillating magnetic field induces electric currents inside the cookware, which results in heating of the cookware due to the electrical resistance of the cookware. The purpose of the cookware being constructed from ferromagnetic material is so that high eddy currents in the cookware are produced in the presence of the oscillating magnetic field, resulting in high energy dissipation, and thus, sufficient heating of the cookware.
Induction geysers (water heaters) represent a new application for the use of induction heating. An induction geyser includes a resonant tank through which rectified power from an AC power source flows, and a fluid tank containing water. The fluid tank is constructed from ferromagnetic material.
When alternating current flows through the resonant tank, when operated in a quasi-resonant mode, eddy currents are induced in the ferromagnetic material of the fluid tank, resulting in the fluid tank heating up, which in turn heats the water.
Using inductive heating as opposed to well-known resistive heating brings a variety of advantages. For example, with resistive heating, the power rating is necessarily dependent on the AC voltage it receives, which may be inconsistent. In addition, for a water heater, resistive heating typically uses a resistive heating element within a fluid tank, and hard water can form white scaling around the resistive heating element, causing degradation in ability to heat the water effectively. Since the heating element in an induction geyser is the ferromagnetic material of the fluid tank itself, this issue is not present. Moreover, since the heating element of the induction geyser is the ferromagnetic material in the fluid tank, the induction geyser is capable of more rapid heating than a water heater relying on resistive heating, as the heating element of the induction geyser has a greater surface area.
This makes induction geysers particularly useful for energy conscious applications, such as developing areas, and for applications where quick heating of water is desired, such as vacation homes in which a water heater will typically only be turned on once the vacationers have arrived.
A known technique for driving the resonant tank of a quasi-resonant induction converter in an induction geyser is to use a low-side drive transistor coupled to pull current from a rectifier 6 through the resonant tank 8. An example of such a circuit 1 is shown in FIG. 1, in which a pulse width modulation (PWM) generator 2 generates a PWM control signal 3 that is applied to a gate driver 4, which generates a gate drive signal 5 that is applied to the gate of an insulated gate bipolar transistor (IGBT) T0. A rectifier 6 rectifies AC power from an AC Mains line. The IGBT transistor T0 has its collector coupled to the resonant tank and its emitter coupled to ground, and serves to pull current from the rectifier 6 through the resonant tank 8.
Although the circuit 1 of FIG. 1 allows for the realization of an induction geyser, such induction geysers are not tuned to deliver maximum power to their fluid tanks, so that they require a longer period of time to heat their water than would be possible with maximum power delivery. This lack of tuning to deliver maximum power to the fluid tank is done for a variety of reasons.
For example, in developing countries, the voltage of the AC power source (e.g. AC Mains) may be inconsistent. In addition, process and material variation in realizing the resonant tank and fluid tank may result in less than optimal eddy current generation. Moreover, magnetic coupling between the resonant tank and the fluid tank may be inconsistent. In addition, this existing solution lacks a comprehensive safety mechanism to protect the IGBT. Therefore, for all these reasons the IGBT is operated so as to maintain high safety margins, and the converter is thus operated at a low power, resulting in the longer period of time to heat water. Also, the protections to the IGBT provided by the existing solution are not foolproof.
It would be desirable to maximize, or come close to maximizing, power output of the converter while properly protecting the IGBT. Therefore, the development of further control circuitry for quasi-resonant converters used in induction geysers and in other applications is necessary.